Preheat the oven to a temperature of 350°F (175°C). Proceed to grease a baking dish measuring 9" x 13" and keep it aside for later use.

Pour olive oil into a sizable pan or skillet. Heat the oil over medium-high heat. Subsequently, introduce the finely diced onions, sautéing them until they become translucent, roughly 3 minutes. Incorporate the minced garlic and continue to sauté for an additional minute, ensuring the garlic releases its aroma but does not burn. Introduce the lean ground beef to the pan, ensuring it is broken into fine crumbles as it browns.

After browning, drain the excess fat from the beef. Return the beef to the pan. Season with 1 teaspoon of the designated Italian seasoning. Integrate this seasoned beef with 2 cups of the Marinara sauce. Once thoroughly mixed, set aside.

In parallel to preparing the beef, boil the spaghetti noodles in water seasoned with Kosher salt. It is advised to boil the spaghetti for a duration 3 minutes less than the al dente recommendation on its packaging.

Once boiled, drain the spaghetti. Return the drained spaghetti to its pot, and amalgamate it with 1 cup of Marinara sauce. Once thoroughly combined, set it aside for subsequent use.

In a separate medium-sized bowl, combine the room-temperature cream cheese, ricotta cheese, garlic powder, and the remaining teaspoon of Italian seasoning. Ensure these ingredients are mixed thoroughly.

To assemble the casserole, commence by placing half of the previously prepared spaghetti in a uniform layer within the 9" x 13" baking dish. Carefully spread the mixed cheese preparation over the initial layer of spaghetti. Then, introduce the remaining spaghetti in another even layer atop the cheese. Thereafter, evenly distribute the beef and sauce mixture over this layer. Finally, garnish the assembly with a generous layer of freshly grated mozzarella cheese.

Place the baking dish in the preheated oven, allowing it to bake for a duration of 30 minutes, or until the top layer turns a pleasant shade of golden brown. Should a darker hue be desired, briefly place the dish under a broiler for 2-3 minutes, monitoring closely to prevent the cheese from charring. Once cooked to satisfaction, remove from the oven and serve in a timely manner.